Automatic Antenna Tuning Units (ATUs) are well known in the field of High Frequency (HF) (2–30 MHz) radio transceiver design, where the radio and antenna must be designed to operate over wide frequency bands. High frequency ATUs are typically designed to handle significant power levels, such as one kilowatt, and utilize, for example, motor-driven variable capacitors and inductors that are capable of withstanding many kilovolts of RF (Radio Frequency) voltage and many amps of current. Such ATUs are, however, bulky, expensive, and operationally slow.
Typically, high frequency ATUs operate by initiating a tuning phase after a change to the transmitter operating frequency. During the tuning phase, transmission of a possibly unmodulated carrier occurs and the ATU searches for the minimum VSWR (Voltage Standing Wave Ratio) condition. The status of the ATU is then frozen until the next frequency change or until manual reinitiation of the tuning phase.
The VSWR is a ratio of the amplitude of the electric field or voltage at a voltage minimum to that at an adjacent maximum in a stationary wave system. The VSWR value is an expression of the impedance mismatch in the antenna resulting in signal reflection. The higher the value of the VSWR at a given frequency, the more signal loss that occurs as a result of signal reflection. Thus, it is desirable to have a low value for VSWR within a given frequency band. The lower the VSWR value, the less signal loss that occurs, resulting in improved signal transmission.
In other frequency bands, for example, the Very High Frequency (VHF) (30–100 Mhz) amps, the antenna Q (quality)-factors are generally much less then high frequency antenna Q-factors. Thus, very high frequency ATUs can be made using step-tuned inductors or capacitors and associated relays or PIN diodes for switching in or out the correct combination of capacitors. Typically, a set of switch commands are predetermined for each frequency channel and stored as binary bytes in a read-only memory. These switch commands switch in or out the correct combination of matching components. Thus, upon a change in frequency, the stored switch commands can be retrieved from the memory for the new frequency channel and used to operate the various switches. This type of antenna matching permits frequency hopping radios to be made where the antenna is tuned for each new hop frequency. The antenna tuning typically occurs in a time period between each new frequency hop during which no transmission takes place. This time period is typically known as the guard time. However, in prior art frequency hopping, a mismatch observed on a previous frequency hop is not correlated with the mismatch on a subsequent frequency hop. The stored tuning commands for the subsequent frequency hop are simply retrieved from memory to switch in or out the correct combination of matching components. There is no adaptive correction based on previous mismatches.
In current applications, wireless communication devices, such as cellular phones, must operate at various frequencies in the RF, HF, VHF, UHF (Ultra-High Frequency) or low microwave bands to transmit and receive signals in, for example, a Time Division Multiple Access (TDMA) network. Without a tuning device, the antenna impedance is a function of the operating frequency and may also vary substantially depending upon the proximity of the antenna to the human body. Therefore, it may not be sufficient to determine fixed matching commands for the various channel frequencies, such as in conventional frequency hoping radios, due to the varying proximity of the cellular phone, and hence the antenna, to a user's body. Further, the proximity of the cellular phone to the user's body may vary during a call, necessitating the detection and correction of a resultant impedance change without interrupting the call or otherwise distorting the signal. There is therefore a need for a very small, low cost, adaptive antenna matching device that is capable of operating continuously during normal transceiver use.
The present invention is directed toward overcoming one or more of the above-mentioned problems.